About measurement of extreme values of parameters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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In practice, sometimes one has to face the problem of determining the values of resistances and capacitances that exceed the upper limit of measurement of devices. For such cases, I recommend using a reference resistor or capacitor, the parameter value of which is close to the maximum limit. In this case, the measured resistor is connected in parallel with the reference one, and the measured capacitor is connected in series with the reference one.
As is known from the school physics course, the resulting value of capacitance or resistance will be less than the value of capacitance and resistance of each of the elements being tested. Transforming the well-known formula for resistors connected in parallel, we get a simple expression
Rx = Rmeas x R1/(R1 - Rmeas),
where Rx is the resistance of the measured resistor; Rmeas - instrument readings; R1 is the resistance of the reference resistor.
For the capacitance of a capacitor, a similar formula has the form
Cx \u1d Cism x C1 / (CXNUMX - Cism).
This method is quite accurate provided that the resistance of the measured resistor and the resistance of the standard are of the same order of magnitude. But even with a tenfold difference in the values of resistance and capacitance, it gives an error that is permissible in amateur practice.
So, when measuring a resistance of 10 MΩ on a digital multimeter with an upper limit of 2 MΩ and a reference resistance of 1,6 MΩ, the error was 3%.
Author: I. Tsaplin, Krasnodar
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